For better understanding the following annotates are used to explain the following technical terms appeared in the specification:
EN-GJS-400-18U-LT or EN-GJS-350-22U-LT, trademarks of spherical graphite iron;
QT400-18AL or QT350-22AL, Chinese engineering standards of spherical graphite iron;
EN DIN12680 Level 2 or Level 3 of the UT tests, Chinese engineering standards of non-destructive ultrasonic testing (EN DIN 12680) second level or third level;
EN DIN 1369 Level 3 of the MT tests, a Chinese engineering standard of non-destructive magnetic particle testing (EN DIN 1369) third level.
Currently the price level of energy resources is high, and the dependence on oil leads to a concerning oil crisis. While this is an important global issue, wind energy as a sustainable and green energy source is being promoted all over the world to be an alternative method for electrical generation. Megawatt wind turbines have become the mainstream product in the wind energy industry, and will be an important source of electricity in the electric grid.
The base of the high-power turbines is a crucial part, not only because its working environment is severe, but it needs to support huge dynamic and static load and needs to have a lifespan of at least 20 years as well. This requires high quality of the casting surface and the firmness of the inner structures, and to lower shrinkage cavity and disperse shrinkage. Currently common sand molding processes do not guarantee the casting quality of the bottom surface of the base. All areas of the base are required to go through non-destructive Ultrasonic Testing (UT) and need to meet the standards of EN DIN12680 Level 2 or Level 3 of the UT tests. The essential areas of the base needs to go through Magnetic Particle Testing (MT) and needs to meet the standards of EN DIN 1369 Level 3. The surface cannot have defects thicker than the thickness of the wall and may not have welding repairs. The roughness of the surface needs to be Ra 50-100.
In addition, to meet high performance requirements, the material of the base is spherical graphite iron castings that work under low-temperature conditions. Common trademarks are EN-GJS-400-18U-LT or EN-GJS-350-22U-LT, and structurally the base castings are big complex parts, whose outer radius is 4-5 m, wall thickness 60-200 mm and weight 10-25 ton. The structure of the base is a big flat-bottom part with thick walls, used to support the gear case, contour and other parts. Spherical graphite iron has better performance than other materials, but it is harder to control in the casting process and is more likely to have shrinkage cavity, disperse shrinkage, and oxidation dregs. These problems are more pronounced for big, complex ductile iron with large cross-section areas, and are unlikely to meet the standards of EN DIN12680 Level 2 or Level 3 of the UT tests and EN DIN 1369 Level 3 of the MT tests. Particularly, since the bottom surface of the base has a large area, common sand molding procedure does not ensure good casting quality of the bottom surface. In order to lessen and eliminate disperse shrinkage, the common treatment was to put chills in the hot spots on the base. This method is helpful, but the chills are apt to react with the iron liquid, which produces oxidation dregs and air holes, making the magnetic particles ineffective for detecting defects. The usage of the chills further complicates the sand molding procedure, which lowers the efficiency of the technique. CN200710144925.3 proposes a fast compelling cooling system that is suitable for producing thick and big cross-section castings. This system targets on solving the problem of low dynamic performance of the thick and big cross-section ductile iron, which due to the distortion and floatation of the spherical graphite cast iron, and disperse shrinkage, cavity, and other defects as a result of the long-time crystallization of the liquid iron. The solution is as follows: the outlet of the liquid nitrogen tank is connected to one end of the low-temperature close valve, the other end connected to the inlet of the liquid nitrogen tank. One side of the liquid nitrogen cooler is implemented in the cavity of the cooling core tube, and the other side of the liquid nitrogen cooler is connected to a nozzle. The said method is very costly.
In order to reduce and eliminate disperse shrinkage and meet the standards of EN DIN12680 Level 2 or Level 3, another method that is commonly used is to place many insulating and exothermic risers at the highest place in the sand mold to make up for the shrinks. However, using insulating and exothermic risers lowers the production rate of the technique, and increases the cost of production, which lowers the overall profit of the casting factories.
The base is made by ductile iron, the production of which is prone to first and second oxidation dregs. To decrease the dregs on the surface, the oversea approach is to use high quality furnace charge and electric furnace refining. However, in China raw materials like pig iron have lower quality than the oversea standards, and the smelt technology is also behind, therefore the dregs on the surfaces are hard to get rid of, and the products are unlikely to meet the standards of EN DIN 1690 Level 3 of the MT tests.
CN200510022689.9 −40° C. Low Temperature As-Cast of Ni-Free Ductile Iron for Casting the Base of Megawatt Wind Power Unit is another patent from this applicant. The ingredients are C 3.6-3.9%, Si 1.7-2.5%, Mn 0.1-0.3%, no Ni, Mg residue 0.045-0.07%. The remainder is iron and impurities, among which P<0.04%, and S<0.02%. The low temperature as-cast of Ni-free ductile iron is obtained by adding nodularizer and nucleating agent and post inoculation. The casting technique after nodularization is as follows: at 1300-1380° C. the liquid mixture is poured into the casting mold and it is slowly cooled down in the mold to below 400° C., then taken out of the mold. This method does not guarantee the high qualification rate of the product.